HYPERPLASTIC PHLOEM IN SUGARBEET LEAVES INFECTED WITH THE BEET CURLY TOP VIRUS

Electron microscopy of sugarbeet leaves infected with the beet curly top virus confirmed earlier findings by light microscopy that the hyperplastic phloem consists mainly of sieve elements that are more or less abnormal in structure. Some parenchyma cells and occasional companion cells may be present. The hyperplastic phloem develops in the place of normal phloem and sometimes in the adjacent ground tissue and the xylem. The sieve elements vary in shape and may be haphazardly arranged. The protoplasts of the sieve elements have the usual characteristics of this type of cell. The sieve element plastids develop from chloroplasts if the hyperplasia occurs in chloroplast-containing parenchyma cells. The cell walls have sieve areas that often are less well differentiated than those of normal sieve elements. The hyperplastic growth in the phloem of curly top diseased plants is discussed with reference to plant tumors induced by certain other plant viruses.     

A study of the source of virus in the Pholem exudate appearing on leaves of Amscinckia infected with the beet curly top virus

Abstract Leaves of Amsinckia douglasiana discharging phloem exudate after infection with the beet curly top virus (BCTV) were studied with the electron microscope. Infected tissue differed from the noninfected in having much hyperplastic phloem characterized by abnormally high proportion of sieve elements, scarcity of companion cells, degenerating parenchyma cells, and some unusually large intercellular spaces. Many spaces contained amorphous debris. Particles resembling BCTV were discernible within the debris. Such particles were encountered also in the debris trapped between stomatal guard cells. Since the phloem exudate excreted from leaves of BCTV-infected plants contains virus particles, and since the virus is found extremely rarely in sieve elements, we suggest (1) that most of BCTV particles apparently released into intercellular spaces are derived from degenerating parenchyma cells in which the virus had multiplied; (2) that the exudate is derived from sieve elements of the hyper-plastic phloem in which the normal functional control by companion cells is lacking; (3) that the exudate leaks from the nontransporting sieve elements through cell walls into intercellular spaces and carries the virus to the outside. Initially, stomata may serve as exits for the infectious exudate, but subsequently ruptures in the epidermis are involved.     

Secondary phloem anatomy of Cycadeoidea (Bennettitales)

Secondary phloem anatomy of several species of Cycadeoidea is described from trunks in the Wieland Collection, Peabody Museum of Natural History. The trunks were collected from the Lakota Formation, Lower Cretaceous, Black Hills of South Dakota. Secondary phloem is extensively developed and consists of alternating, tangential bands of fibers and sieve elements, with rare phloem parenchyma. Uniseriate rays, 2-22 cells high, occur between every one to three files of the axial system. Fibers are long, more than 1200 μm, approximately 26.6-34.2 μm in diameter, and have slit-like apertures on the lateral walls. Sieve elements range from 16-25 μm in diameter and are up to 500 μm long. Elliptical sieve areas appear on both end and radial walls and measure 10 μm across; minute spots, which may represent sieve pores, are present within the sieve areas. Secondary phloem of North American Cycadeoidea is similar in organization (alternating tangential bands) and cell types (sieve cells, fibers, axial parenchyma) to that known in other extant and fossil cycadophytes and some seed ferns. The unusual pattern of cell types and thickness of secondary phloem is discussed in the context of plant habit, phloem efficiency, and potential phylogenetic importance.     

A Rapid Method for Macerating Phloem

Sieve cells and sieve tube members can be macerated from the phloem of various organs of woody and herbaceous species by au-toclaving the tissue in a mild macerating medium. This treatment does not digest the primary walls or the callose deposits on the sieve areas and sieve plates of the sieve elements. These cells can then be recognized by the fluorescence of their callose after staining with aniline blue. Sometimes adjacent sieve elements fail to separate and one can observe details of their junctures.     

PHLOEM ANATOMY IN STAUROPTERIS BISERIATA FROM THE PENNSYLVANIAN OF NORTH AMERICA

Phloem anatomy in the coenopterid fern Stauropteris biseriata is detailed from Lower-Middle Pennsylvanian coal ball specimens from eastern Kentucky. Axes exhibit a cruciate-shaped xylem trace in transverse section. Phloem tissue completely surrounds the xylem, but is more extensively developed in the embayments between the xylem arms. Phloem is composed of elongate conducting elements with a few scattered parenchyma cells. Large and small sieve cells are present, with larger ones occurring in the embayments within the primary plane of symmetry of the axes. Large elements are approximately twice the diameter of the smaller sieve elements. Oval sieve areas and pores have been observed on lateral and oblique end walls of both large and small elements. The structure and composition of Stauropteris phloem is discussed in relationship to the available information on phloem anatomy in other fossil cryptogams.     

The Phloem of Nelumbo nucifera Gaertn

In common with other aquatic angiosperms, Nelumbo nucifera Gaertn.has a relatively strongly developed phloem tissue. The vascularsystem consists of discrete collateral bundles in which no cambiumdevelops and the phloem and xylem are separated by a narrowlayer of parenchyma cells. The phloem consists of sieve elements,companion cells, and phloem parenchyma cells. The sieve elementshave transverse end walls with simple sieve plates. The cellsattain considerable width in the late phloem (metaphloem). Thecompanion cells are in vertical strands. In the early phloem(protophloem) of large bundles the sieve tubes and companioncells are eventually obliterated. The parenchyma cells alsoform vertical strands which may contain tannin cells. Some parenchymacells and companion cells are binucleate. The sieve elementsshow ultrastructural features common for these cells in dicotyledons.At maturity, they lack nuclei, ribosomes, and tonoplasts, butretain a plasmalemma, mitochondria, and plastids. The latterare poorly differentiated and form starch. The endoplasmic reticulumis in part stacked, in part it forms a network next to the plasmalemma.The P-protein occurs in two forms. One consists of tubules notassembled in any specific type of array. The other, possiblycomposed of much extended tubules, is assembled in crystallineaggregates which are retained as such in mature cells. The sieveplate pores are lined with callose and plasmalemma. The lateralwalls are relatively thin and the nacreous layer varies in degreeof distinctness.     

杨柳科(Salicaceae)7种植物次生韧皮部的比较研究

本文研究和比较了杨柳科2属7种植物次生韧皮部解剖结构。结果表明:(1)杨属和柳属植物在次生初皮部解剖上有某些共同特征:次生韧皮部具有明显分层现象;韧皮纤维和含晶细胞与筛管分子、伴胞和韧皮薄壁组织细胞是切向带相间排列;筛管分子均为复筛板,端壁倾斜平均含有7-8个筛域。(2)两属植物在射线和晶体类型上有明显区别:柳属植物次生韧皮部无石细胞;杨属植物不具功能韧皮部中含有石细胞。(3)两属植物均有一些较为原始的韧皮部解剖特征。     

Sieve Cell Differentiation; Accurate Determination by Polarized Light

Differentiating sieve cells can be qualitatively and quantitatively determined in white pine or other species of plants with phloem cells possessing nacreous primary walls or thickened secondary walls. Transverse sections from stained and unstained preparations of white pine examined in polarized light reveal a distinct zone of birefringent sieve cells situated between the cambial zone and layer of seasonal phloem parenchyma. The deposition of secondary walls in sieve cells in pine and their unequivocal recognition in polarized light presents a simple, effective means for detecting newly differentiated sieve cells and for quantitatively estimating their production during an experimental period.     

Wood Anatomy and the Development of Interxylary Phloem of Ipomoea hederifolia Linn. (Convolvulaceae)

In Ipomoea hederifolia Linn., stems increase in thickness by forming successive rings of cambia. With the increase in stem diameter, the first ring of cambium also gives rise to thin-walled parenchymatous islands along with thick-walled xylem derivatives to its inner side. The size of these islands increases (both radially and tangentially) gradually with the increase in stem diameter. In pencil-thick stems, that is, before the differentiation of a second ring of cambium, some of the parenchyma cells within these islands differentiate into interxylary phloem. Although all successive cambia forms secondary phloem continuously, simultaneous development of interxylary phloem was observed in the innermost successive ring of xylem. In the mature stems, thick-walled parenchyma cells formed at the beginning of secondary growth underwent dedifferentiation and led to the formation of phloem derivatives. Structurally, sieve tube elements showed both simple sieve plates on transverse to slightly oblique end walls and compound sieve plates on the oblique end walls with poorly developed lateral sieve areas. Isolated or groups of two to three sieve elements were noticed in the rays of secondary phloem. They possessed simple sieve plates with distinct companion cells at their corners. The length of these elements was more or less similar to that of ray parenchyma cells but their diameter was slightly less. Similarly, in the secondary xylem, perforated ray cells were noticed in the innermost xylem ring. They were larger than the adjacent ray cells and possessed oval to circular simple perforation plates. The structures of interxylary phloem, perforated ray cells, and ray sieve elements are described in detail.     

水松的次生韧皮部解剖及其系统位置的讨论

在光学显微镜和扫描电子显微镜下观察,水松茎次生韧皮部的主要特征为：韧皮部由轴向系统和径向系统组成。轴向系统由筛胞、韧皮薄壁组织细胞、蛋白细胞和韧皮纤维组成,径向系统由韧皮射线组成。在横切面上,轴向系统的各组成分子以单层切向带交替有规律的排列,其排列顺序为：筛胞－韧皮薄壁组织细胞－韧皮纤维-筛胞。筛胞的径向壁上嵌埋有草酸钙结晶,韧皮纤维仅一种类型,韧皮射线同型、单列。根据水松茎次生韧皮部的解剖研究,并与杉科其它各属的有关资料进行比较,我们认为：水松属与水杉属和落羽杉属有较近的亲缘关系。     

Anatomy of Secondary Phloem of Glyptostrobus in Relation to Its Systematic Position

A anatomical characters of secondary phloem in Glyptostrobus pensilis (Staunt.)Koch were observed by means of both light and scanning electron microscopy(SEM). The secondary phloem is composed of axial and radial systems. In the axial systems, the phloem consists of sieve cells, phloem parenchyma cells, albuminous cell and phloem fibers. In the radial systems, it consists of phloem rays. The alternate arrangement of different cells in cross section results in tangential bands. The sequence of radial arrangement follows the pattern of sieve cells, phloem parenchyma cells, sieve cells and phloem fibers, sieve cells. Many crystals of calbium oxalate are embedded in the radial walls of seive cells. The phloem fibers are of only one type. The phloem rays are homogeneous, uniseriate. According to the anatomical characters of secondary phloem of Glyptostrobus pensilis (Staunt.)Koch and comparison with the other genera of Taxodiaceae, Glyptostrobus, Metasequoia and Taxodium have close relationships.     

Anatomy of Gymnosperms Endemic to China,II. Taiwania flousiana Gaussen (Taxodiaceae)

Taiwania Hayata contains two species: T.flousiana Gaussen and T. cryptomerioides Hayata, both endemic to China. T. flousiana was investigated with both light and scanning electron microscopes in respect to shoot apex, external and internal surfaces of leaf cuticle, primary leaf, juvenal and mature leaves, young stem, secondary phloem and wood of stem, etc, It is shown that the shoot apex consists of the following five regions: (1) the apical initials; (2) the protoderm, (3) the subapical moher cells;. (4) the peripheral meristem, and (5) the pith mother cells. The periclinal and anticlinal division of the apical initials takes place with approximately equal frequency. The juvenal leaf is nearly triangular or crescent-shaped in cross section and belongs to the leaf type II. The mature leaf is quadrangular in cross section (the leaf type I). There are a progressive series of changes in size and shape of the leaf cross section. The stoma of the mature leaf is amphicyclic and occasionally tricyclic. The crystals in the juvenal leaf cuticle are more abundant than those in the mature leaf cuticle. The transfusion tissue conforms to the Cupressus type. The structure of juvenal leaf is the nearest to that in Cunninghamia unicanaliculata D. Y. Wang et H. L. Liu, while the mature leaf is similar to that of the Cryptomeria. Sclerenchymatous cells of the hypodermis in the young stem comprise simple layers and are arranged discontinuously. No primary fibers are found in the primary phloem. Medullary sheath is present between the primary xylem and the pith. There are some sclereids in the pith. The secondary phloem of the stem consists of regularly alternate tangential layers of cells in such a sequence: sieve cells, phloem parenchyma cells, sieve cells, phloem fibers, sieve cells. The phloem fiber may be divided into thick-walled and thin-walled phloem fiber. The crystals of calcium oxalate in the radial walls of sieve cells are abundant. Homogeneous phloem rays are uniseriate or partly biseriate, 1-48 (2-13) cells high, and of 26-31 strips per square mm. Growth rings of the wood in Taiwania are distinct. The bordered pits on the radial walls of early wood tracheids are usually uniseriate, occasionally paired and opposite pitting. Wood parenchyma is present, and its cells contain brown resin substances. Their end walls are smooth, lacking nodular thickenings. Wood rays are homogeneous. Cross-field pits are cupressoid. Resin canals are absent. Based on the anatomy of Taiwania and comparison with the other genera of Taxodiaceae, the authors consider the establishment of Taiwaniaceae not reasonable, but rather support the view that the genus is better placed between Cuninghamia and Arthrotaxis in Taxodiaceae.     

Localization of Phloem Oxidases

Among oxidases, cytochrome oxidase has been localized in mitochondria of all phloem cells, catalase has been visualized in parenchyma peroxisomes and peroxidase has been localized in cell walls and in several cell organelles. In angiosperms, peroxidase is present in all phloem cell walls; it is sensitive to cyanide inhibition excepted in sieve areas and around plasmodesmata between sieve tubes and companion cells. In some species, this cyanide resistant oxidasic activity can be localized without exogenous H₂O₂. Peroxidase is localized on ribosomes, inside vacuoles, on the tonoplast and often on the plasmalemma in companion cells and differentiating sieve elements. In young sieve cells some dictyosomes can exhibit a strong peroxidasic activity. In mature parenchyma cells peroxidase can be associated with ER cisternae but not with vacuoles.     

竹子节部“韧皮部结”的发育与超微结构

研究了中国最为重要的经济竹种毛竹（Ｐｈｙｌｌｏｓｔａｃｈｙｓ ｅｄｕｌｉｓ （Ｃａｒｒ．）Ｈ．ｄｅ Ｌｅｈａｉｅ）节部“韧皮部结”的个体发育、构成该结构细胞的形态学特征及其超微结构,探讨了该结构可能的生理功能。“韧皮部结”的发育直接来源于原形成层,发生在维管束分叉处,一般成对出现。“韧皮部结”外形呈纺锤体状,一般由４～６层细胞形成叠生构造。构成“韧皮部结”的细胞可以区分为两类,一类是位于纺锤体中部     

Sieve elements rapidly develop ‘nacreous walls’ following injury − a common wounding response?

Thick glistening cell walls occur in sieve tubes of all major land plant taxa. Historically, these ‘nacreous walls’ have been considered a diagnostic feature of sieve elements; they represent a conundrum, though, in the context of the widely accepted pressure–flow theory as they severely constrict sieve tubes. We employed the cucurbit Gerrardanthus macrorhizus as a model to study nacreous walls in sieve elements by standard and in situ confocal microscopy and electron microscopy, focusing on changes in functional sieve tubes that occur when prepared for microscopic observation. Over 90% of sieve elements in tissue sections processed for microscopy by standard methods exhibit nacreous walls. Sieve elements in whole, live plants that were actively transporting as shown by phloem‐mobile tracers, lacked nacreous walls and exhibited open lumina of circular cross‐sections instead, an appropriate structure for Münch‐type mass flow of the cell contents. Puncturing of transporting sieve elements with micropipettes triggered the rapid (<1 min) development of nacreous walls that occluded the cell lumen almost completely. We conclude that nacreous walls are preparation artefacts rather than structural features of transporting sieve elements. Nacreous walls in land plants resemble the reversibly swellable walls found in various algae, suggesting that they may function in turgor buffering, the amelioration of osmotic stress, wounding‐induced sieve tube occlusion, and possibly local defence responses of the phloem.     

PHLOEM OF SPHENOPHYLLUM

The phloem of most fossil plants, including that of Sphenophyllum, is very poorly known. Sphenophyllum was a relatively small type of fossil arthrophyte with jointed stems bearing whorls of leaves ranging in form from wedge or fan-shaped to bifid, to linear. The aerial stem systems of the plant exhibited determinate growth involving progressive reduction in the dimensions of the stem primary bodies, fewer leaves per whorl, and smaller and simpler leaves distally. The primary phloem occurs in three areas alternating in position with the arms of the triarch centrally placed primary xylem. Cells of the primary phloem, presumably sieve elements, are axially elongate with horizontal to slightly tapered end walls. In larger stems with abundant secondary xylem and secondary cortex or periderm, a zone of secondary phloem occurs whose structure varies in the three areas opposite the arms of the primary xylem, as opposed to the three areas lying opposite the concave sides of the primary xylem. The axial system of the secondary phloem consists of vertical series of sieve elements with horizontal end walls. In the areas opposite the protoxylem the parenchyma is present as a prominent ray system showing dilation peripherally. Sieve elements in the areas opposite the protoxylem arms have relatively small diameters. In the areas between the protoxylem poles the secondary phloem sieve elements have large diameters and are less obviously in radial files, while the parenchyma resembles that of the secondary xylem in these areas in that it consists of strands of cells extending both radially and tangentially. An actively meristematic vascular cambium has not been found, indicating that this layer changed histologically after the cessation of growth in the determinate aerial stem systems and was replaced by a post-meristematic parenchyma sheath made up of axially elongate parenchyma lacking cells indicative of being either fusiform or ray initials. A phellogen arose early in development in a tissue believed to represent pericycle and produced tissue comparable to phellem externally. Normally, derivatives of the phellogen underwent one division prior to the maturation of the cells. Concentric bands of cells with dark contents apparently represent secretory tissue in the periderm and cell arrangements indicate that a single persistent phellogen was present. Sphenophyllum is compared with other arthrophytes as to phloem structure and is at present the best documented example of a plant with a functionally bifacial vascular cambium in any exclusively non-seed group of vascular plants.     

STRUCTURE AND DEVELOPMENT OF THE SIEVE ELEMENT IN THE STEM OF LYCOPODIUM LUCIDULUM

Stem tissue of Lycopodium lucidulum Michx. was fixed in glutaraldehyde and postfixed in osmium tetroxide for electron microscopy. Although their protoplasts contain similar components, immature sieve elements can be distinguished from parenchymatous elements of the phloem at an early stage by their thick walls and correspondingly high population of dictyosomes and dictyosome vesicles. Late in maturation the sieve-element walls undergo a reduction in thickness, apparently due to an “erosion” or hydrolysis of wall material. At maturity, the plasmalemma-lined sieve elements contain plastids with a system of much convoluted inner membranes, mitochondria, and remnants of nuclei. Although the endoplasmic reticulum (ER) in most mature sieve elements was vesiculate, in the better preserved ones the ER formed a tubular network closely appressed to the plasmalemma. The sieve elements lack refractive spherules and P-protein. The protoplasts of contiguous sieve elements are connected with one another by pores of variable diameter, aggregated in sieve areas. As there is no consistent difference between pore size in end and lateral walls these elements are considered as sieve cells.     

Fine structure,pattern of division,and course of wound phloem in Coleus blumei

The wound phloem bridges which have developed six days after interrupting an internodal vascular bundle contain wound sieve-elements, companion cells, and phloem parenchyma cells. An analysis of the meristematic activity responding to the wounding clearly demonstrates that three consecutive divisions are prerequisite to the formation of phloem mother-cells. Companion cells are obligatory sister cells of wound sieve-elements, connected to the latter by specific plasmatic strands and provided with a dense protoplast. Six days after wounding most of the wound sieve-elements are still at a nucleate state of development, but already have characteristic P-protein bodies and plastids containing sieve-element starch. Their cytoplasmic differentiation corresponds to the changes recorded during maturation of ordinary sieve elements. Sieve-plate pores penetrate through preexisting parenchyma cell walls, only, and develop from primary pitfield-plasmodesmata. Wound sieve-elements do not connect to preexisting bundle sieve-elements, they open a new tier of young sieve elements produced by cambial activity.     

A Study of Stelar Ultrastructure in the Heterosporous Water Fern Marsilea quadrifolia L

Sieve tube elements occur in the rhizomes and petioles of Marsileaquadrifolia. These are either thick walled with compound sieveplates in oblique end walls or thin walled with simple sieveplates in transverse end walls. Vessels are restricted to themetaxylem in the roots where the phloem contains sieve cellsonly. The sieve pores are invariably callose lined and as inother pteridophytes, excepting the Lycopsida, refractive spherulesare ubiquitous in the sieve elements of Marsilea. The luminaof the protoxylem tracheary elements in the rhizomes and petiolesare occluded by tyloses but probably remain functional in theroots. Pericycle cells backing on to the root protoxylem armspossess wall ingrowths. Transfer cells are however absent fromthe vascular tissue of the rhizomes and leaves. It is suggestedthat their presence in the root pericycle is related to theretrieval of ions from the xylem sap which may be particularlycritical in water plants. The incidence of transfer cells incryptogams appears to be far more sporadic than in angiosperms.The root endodermis of Marsilea possesses a casparian stripand abundant vacuolar tannin deposits. Plasmalemmasomes arenumerous adjacent to the pericycle transfer cells. vascular ultrastructure, Marsilea quadrifolia L, transfer cells, sieve tube elements, tyloses     

THE PHLOEM OF ETAPTERIS LECLERCQII AND BOTRYOPTERIS TRIDENTATA

The phloem of Etapteris leclercqii and Botryopteris tridentata petioles is described from Lower Pennsylvanian coal balls. Petioles of B. tridentata are characterized in transverse section by an omega-shaped xylem trace, a phloem zone which extends from 2-10 cells in width, and 2-parted cortex. Etapteris leclercqii petioles exhibit a 4–9 cell-wide phloem zone surrounding the central clepsydroid xylem mass, and a 3-parted cortex. In both taxa a 1–2 cell layer parenchyma sheath separates the xylem from the extra-xylary tissues. The phloem of both species consists of sieve elements that average about 20 μm in diam by 200 μm in length in Botryopteris, and 100 μm in length in Etapteris, with horizontal-slightly oblique end walls. In transmitted light, the radial walls of the sieve elements form an irregular reticulate pattern enclosing elliptical lighter areas. With the scanning electron microscope, these areas appear as horizontal-slightly oblique furrows on the cell wall, with many small indentations lining the furrows. These indentations, because of their regular occurrence and size (from a few fractions of a micron up to 1.0 μm in diam), are interpreted as sieve pores, and the elliptical areas that enclose them as sieve areas. The phloem of E. leclercqii and B. tridentata is compared with that described for other fossil genera and with that of extant ferns.